High Impedance in the circuit somewhere. Volt drop caused by a fault in the circuit is Impedance (Resistance in this case) times current squared, so when the only load is the meter, current is very low and, therefore, volt drop is very low, so you see the 12V. When you insert the bulb (which wants to draw more current), the volt drop across the impedance goes up, and leaves nothing for the bulb.
So, you are looking for a corroded connection or, in view of the age of the car, one that is loose or a cable that has been damaged. Measure the voltage between the bulb +ve and a known good earth, with and without the bulb inserted. If it varies, that fault is between the bulb -ve and earth, if not, it's in the supply. If it's in the supply, trace the wiring back and check the voltage to earth at various points with and without the bulb. When you get to somewhere that the readings are the same you are "upstream" of the fault. As Neil points out, you'll probably find that the wiring traces back to a CANBUS module. If the voltage out of the module varies with load, then you may well have an expensive problem.....! Take more advice at that point - let's hope it's simpler than that! Hope that helps Dave. On Dec 28, 6:47 pm, barnacle <[email protected]> wrote: > Bad local earth., at least traditionally - but be aware that modern > Fiats use a lot of canbus to turn things on and off, and there may be > no connection between the switch and the bulb. > > Neil > > On Dec 28, 6:45 pm, "Susan and Martin Scott" > > > > > > > > <[email protected]> wrote: > > Here's a puzzler for the holiday!!!!! > > Boot light on our 2yr old fiat isn't working..... > > warning on dash appears if boot is opened, therefore switch is working > > tried a new bulb, still not working........ > > put meter on contacts have 12V........ > > If meter probes are in place, showing 12V, putting bulb to probes reduces > > voltage reading to 0 on meter, but bulb doesn't light. > > Can only think I have a VERY low amp current at 12V, that's not enough to > > light the bulb?? > > > Any pointers gratefully received before I go mad or buy a torch.. > > Martin -- You received this message because you are subscribed to the Google Groups "Quantum Owners Group" group. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/quantumowners?hl=en IMPORTANT NOTE: All information presented herewith is provided on an "As Is" basis, without warranty or the implication thereof. Neither the Quantum Owners Club nor the individuals associated with the Quantum Owners Club or in the preparation of the above information shall have any liability to any person or entity with respect to liability, loss, or damage caused or alleged to be caused directly or indirectly by the instructions contained within this or related message(s).
